diff --git a/web/firefox_print_service.js b/web/firefox_print_service.js
index 37576f660..ed893c4aa 100644
--- a/web/firefox_print_service.js
+++ b/web/firefox_print_service.js
@@ -146,6 +146,27 @@ FirefoxPrintService.prototype = {
     const body = document.querySelector("body");
     body.setAttribute("data-pdfjsprinting", true);
 
+    const hasEqualPageSizes = this.pagesOverview.every(function (size) {
+      return (
+        size.width === this.pagesOverview[0].width &&
+        size.height === this.pagesOverview[0].height
+      );
+    }, this);
+    if (!hasEqualPageSizes) {
+      console.warn(
+        "Not all pages have the same size. The printed " +
+          "result may be incorrect!"
+      );
+    }
+
+    // Insert a @page + size rule to make sure that the page size is correctly
+    // set. Note that we assume that all pages have the same size, because
+    // variable-size pages are scaled down to the initial page size in Firefox.
+    this.pageStyleSheet = document.createElement("style");
+    const pageSize = this.pagesOverview[0];
+    this.pageStyleSheet.textContent = `@page { size: ${pageSize.width}pt ${pageSize.height}pt;}`;
+    body.append(this.pageStyleSheet);
+
     if (pdfDocument.isPureXfa) {
       getXfaHtmlForPrinting(printContainer, pdfDocument);
       return;
@@ -169,6 +190,11 @@ FirefoxPrintService.prototype = {
 
     const body = document.querySelector("body");
     body.removeAttribute("data-pdfjsprinting");
+
+    if (this.pageStyleSheet) {
+      this.pageStyleSheet.remove();
+      this.pageStyleSheet = null;
+    }
   },
 };
 
diff --git a/web/pdf_print_service.js b/web/pdf_print_service.js
index 87cc73c62..e0401f388 100644
--- a/web/pdf_print_service.js
+++ b/web/pdf_print_service.js
@@ -111,9 +111,9 @@ PDFPrintService.prototype = {
     // TODO(robwu): Use named pages when size calculation bugs get resolved
     // (e.g. https://crbug.com/355116) AND when support for named pages is
     // added (http://www.w3.org/TR/css3-page/#using-named-pages).
-    // In browsers where @page + size is not supported (such as Firefox,
-    // https://bugzil.la/851441), the next stylesheet will be ignored and the
-    // user has to select the correct paper size in the UI if wanted.
+    // In browsers where @page + size is not supported, the next stylesheet
+    // will be ignored and the user has to select the correct paper size in
+    // the UI if wanted.
     this.pageStyleSheet = document.createElement("style");
     const pageSize = this.pagesOverview[0];
     this.pageStyleSheet.textContent =
diff --git a/web/pdf_viewer.js b/web/pdf_viewer.js
index 6649a3143..533848517 100644
--- a/web/pdf_viewer.js
+++ b/web/pdf_viewer.js
@@ -1673,21 +1673,27 @@ class PDFViewer {
    * @returns {Array} Array of objects with width/height/rotation fields.
    */
   getPagesOverview() {
+    let initialOrientation;
     return this._pages.map(pageView => {
       const viewport = pageView.pdfPage.getViewport({ scale: 1 });
-
-      if (!this.enablePrintAutoRotate || isPortraitOrientation(viewport)) {
+      const orientation = isPortraitOrientation(viewport);
+      if (initialOrientation === undefined) {
+        initialOrientation = orientation;
+      } else if (
+        this.enablePrintAutoRotate &&
+        orientation !== initialOrientation
+      ) {
+        // Rotate to fit the initial orientation.
         return {
-          width: viewport.width,
-          height: viewport.height,
-          rotation: viewport.rotation,
+          width: viewport.height,
+          height: viewport.width,
+          rotation: (viewport.rotation - 90) % 360,
         };
       }
-      // Landscape orientation.
       return {
-        width: viewport.height,
-        height: viewport.width,
-        rotation: (viewport.rotation - 90) % 360,
+        width: viewport.width,
+        height: viewport.height,
+        rotation: viewport.rotation,
       };
     });
   }